smp: kernel/panic.c - silence warnings

smp: kernel/panic.c - silence warnings

We found these warnings in kernel/panic.c by using sparse tool:
warning: symbol 'panic_smp_self_stop' was not declared.
warning: symbol 'nmi_panic_self_stop' was not declared.
warning: symbol 'crash_smp_send_stop' was not declared.

To avoid them, add declarations for these three functions in
include/linux/smp.h.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: He Ying <[email protected]>
Reported-by: Hulk Robot <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>